This invention relates to an autozero loop for a voice frequency coder-decoder (CODEC).
In transmission systems, CODECs (coder-decoders) with appropriate filters are used for transmission and reception of voice signals. In order to meet the required gain tracking, idle channel noise and signal to distortion specifications, any DC offset in the transmit filter-coder combination of the CODEC must be nulled. Heretofore, an autozero loop has been utilized to achieve this function. This loop takes the long term average of the sign bit of the processed word, and feeds the resulting voltage back into the system using negative feedback to null any DC offset. In CODECs previously disclosed, only a single bandwidth autozero loop has been used to cancel offset of the coder. For example, see "A Two-Chip PCM Codec for Per Channel Applications", James B. Cecil, Edwin M. W. Chow, John A. Flink and James E. Solomon, IEEE International Solid State Circuits Conference, Feb. 1978, pp. 176-177. Also, heretofore the offset associated with the filter in the CODEC is concelled using capacitive coupling between the filter and the encoder. A serious disadvantage with this arrangement is that it requires an off chip capacitor, along with greater pinout connectors, thus resulting in a relatively large and more complicated device.
Another problem arises because an autozero loop by definition has a long time constant associated with it and, therefore, is not functional immediately after the CODEC is enabled (i.e., powered up). This delay impairs system performance for a few seconds immediately after power-up. Also, this delay creates problems in automatic testing of the devices where the total available test time is of the order of several seconds.
It is therefore one object of the present invention to provide an improved autozero loop for a voice frequency CODEC that solves the aforesaid problems.
Other more specific objects of the invention are to provide an autozero loop that: (1) will cancel offsets in the filterencoder combination with the filter direct coupled to the encoder input; (2) will achieve rapid acquisition of offsets in the system immediately after power up; and (3) will use a package with fewer pins compared to systems wherein the coder and filter are capacitor coupled (capacitor coupling requires that the output of the filter and the input of the coder be available off chip through pins on the package).
A further object of the invention is to provide an autozero loop circuit for a voice frequency CODEC that is particularly well adapted for implementation as an integrated circuit device.